Role of Electrochemical Reactions in the Degradation Mechanisms of
AlGaN/GaN HEMTs
1. Department of Materials Science and Engineering, Massachusetts Institute of Technology
2. Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology
Feng Gao 1,2, Bin Lu 2, Carl V. Thompson 1,Jesús del Alamo 2, Tomás Palacios 2
This project is partially funded by the ONR DRIFT MURI program.
Introduction
Wireless Base Station Automotive Electronics
• Excellent for High Power RF & Power Electronics Applications
Energy Conversion2
• AlGaN/GaN HEMTs unique properties
Wide bandgap Ecritical >3MV/cm
Polarization 2DEG density >1013/cm2
High electron mobility: >1500 cm2/V.s
High electron peak velocity: 2.1x107cm/s
2DEG
Ec
Ev
Ef
GaNAlGaN
• Device reliability is one of the greatest obstacles for AlGaN/GaNHEMTs: various degradation modes
Motivation― Reliability Bottleneck
3
[Meneghesso et. al., IEEE Transactions on Device and Materials Reliability, vol.8, no.2, p.332, 2008]
Motivation― Overview of Reliability Phenomena
J. Joh et al., IEEE Electron Dev. Lett., vol. 29, no. 4, 2008.
• Permanent Degradation
U. Chowdhury et al., IEEE Electron Dev. Lett, vol. 29, no.10, 2008.
Surface Pitting
P. Makaram et al., Appl. Phys. Lett., 96, 233509, 2010.
Electrical Degradation
4
Inverse Piezoelectric Effects
J. Joh et al., IEDM 2007. IEEE International, 2007, pp. 385–388.
Motivation― Overview of Reliability Mechanisms
• Physical mechanism in permanent degradation Defect Percolation
Mass-Transport
M. Meneghini et al., Appl. Phys. Lett., vol. 100, no. 3, p. 033505, 2012.
F. Gao et al., Appl. Phys. Lett., vol. 99, no. 22, pp. 223506, 2011.
Water-Assisted Electrochemical Mass Transport in Permanent Degradation
Electrochemical Reaction Mechanism:- Holes generation- Water Diffusion
Goals of This Work
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Experimental Setup― High Vacuum and High Temperature Probe Station
Key features: Vacuum chamber of 1 × 10-7 Torr. Thermal chuck up to 600 °C. Four probes for electrical contact. Two gas lines that can supply the chamber with air, O2, N2, Ar, He and H2.
[email protected] to Prof. Harry L. Tuller8
Impact of Atmosphere on Surface Pitting― Ambient and Vacuum Control-Group
Surface pitting caused by OFF-state electrical stress is significantly reduced in vacuum.
Stressed in ambient air
Stressed in vacuum of 1×10-7 Torr
• Off-state stress test: Vds = 43 V, Vgs = -7 V for 3000s in darkness at RT
SEM Top View AFM Depth ProfileTEM Cross Section
[email protected]. Gao et al., IEEE Trans. Electron Devices, vol. 61, no.2, 20149
Role of Oxygen in Surface Pitting― Ambient and Vacuum Control-Group
Stressed in ambient air
Stressed in vacuum of 1×10-7 Torr
Higher concentration of oxygen (O) is found inside thesurface pits for AlGaN/GaN HEMTs stressed in ambient air.
[email protected]. Gao et al., IEEE Trans. Electron Devices, vol. 61, no.2, 201410
TEM TEM EDX
Influence of Ambient Air
[email protected]. Gao et al., IEEE Trans. Electron Devices, vol. 61, no.2, 201411
• Ambient air more pits with deeper depther
• Ambient air increase of oxygen in pitting area
What gas in the ambient air causes: • surface pitting?• increasing of oxygen concentration in
pitting area?
Ambient air has N2, O2, H2O, CO2
Off-state stress
Impact of Moisture on Surface Pitting― Wet and Dry Control-Group
Stressed in water-saturated gas (Ar)
SEM Top View TEM Cross Section
Stressed in dry gas (Ar)
wet/dry Ar; wet/dry O2 ; wet/dry N2 ; wet/dry CO2 ; wet/dry Air
Water has a major impact on surface pitting & source of O in pitting area
[email protected]. Gao et al., IEEE Trans. Electron Devices, vol. 61, no.2, 201412
Mass Transport― Ga, Al migration
TEM EDX Mapping
Unstressed
TEM EDX mapping: Ga and Al found in the gate region
[email protected]. Gao et al., IEEE Trans. Electron Devices, vol. 61, no.2, 2014
Stressed in ambient for 3000s
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Al
Al
Stressed in vacuum for 3000s
Permanent Electrical Degradation― By Off-State Stress
Off-state stress: Vgs = -7V, Vds = 43V, 3000s at RT in dark 1min UV illumination & 12hr at rest after Off-state to
recover trapping transient Idss and Ig measured at Vgs = 0, Vds =5V
[email protected]. Gao et al., IEEE Trans. Electron Devices, vol. 61, no.2, 201414
Stressed in ambient
Stressed in vacuum
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Permanent Electrical Degradation― Surface Pitting Time Evolution
Stressed in ambient
Permanent Electrical Degradation― Id Degradation Time Evolution
Drain current degradation evolves with the growth of the surface pits over stress time.
• Time Evolution of Drain Current Degradation: Id measured at Vgs = 0 and Vds = 5 V
F. Gao et al., IEEE Trans. Electron Devices, vol. 61, no.2, 201416
H2O surface pitting and source of oxygen in pitting area
Ga, Al migration accompanies surface pitting
Surface pitting & Ga, Al migration permanent electrical degadation
Summary 1
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Al
What is the physical mechanism behind the impact of H2O on the surface pitting ?
Water-assisted Electrochemical Reactions― Corrosion in AlGaN/GaN HEMTs
Reduction of water:
Anodic oxidation of AlGaN:
2H2O + 2e- = 2OH- + H2
2Alx Ga1-xN + 6h+ = 2xAl3+ + 2(1-x)Ga3+ +N2
2xAl3+ + 2(1-x)Ga3+ + 6OH- = xAl2O3 + (1-x)Ga2O3 + 3H2O
Complete Reduction-oxidation (redox) electrochemical reactions:
2Alx Ga1-xN + 3H2O = xAl2O3 + (1-x)Ga2O3 + N2 + H2
An electrochemical cell formed at the drain edge of the gate. e-
h+
H2OOH-
[email protected]. Gao et al., IEEE Trans. Electron Devices, vol. 61, no.2, 201418
GaN
Gate
AlGaN
Water-assisted Electrochemical Reactions― Ga, Al out diffusion
h+
h+
h+h+ Ga 3+Al 3+
AlGa
[email protected]. Gao et al., IEEE Trans. Electron Devices, vol. 61, no.2, 201419
Stressed in ambient for 3000sAl
1. Holes are generated and accumulated at the AlGaN surface.2. Ambient water diffuse through the SiNx passivation and reach the AlGaN surface.
Two necessary conditions:
Source of Holes― Photo-Generated Holes
[email protected]. Gao et al., IEEE Trans. Electron Devices, vol. 61, no.2, 2014
• Stressed under 254-nm UV illumination in ambient air at Vds = 43 V Vgs = -7 V for 3000s at RT
SEM Top View TEM Cross Section
Increasing surface pitting
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Source of Holes― Impact Ionization vs Inter-band Tunneling
Lateral impact ionization
gi
h jEE
j )exp(max
[email protected]. Gao et al., IEEE Trans. Electron Devices, vol. 61, no.2, 201421
Vertical inter-band tunneling
)exp(max
2max E
EEj t
h
Keldysh, Soviet Phys. JETP, vol. 33, no.4, p763, 1958
• Silvaco simulation of Electric Field in AlGaN Layer
Source of Holes― Quantitative Analysis: E-field
[email protected]. Gao et al., IEEE Trans. Electron Devices, vol. 61, no.2, 201422
Emax
Source of Holes― Quantitative Analysis: jh
[email protected]. Gao et al., IEEE Trans. Electron Devices, vol. 61, no.2, 201423
dMt
qNdj Ah
32Alx Ga1-xN + 6h+ = 2xAl3+ + 2(1-x)Ga3+ +N2
• Relationship of Surface Pits and Holes
+Average pits depth measured by AFM
Jh vs Emax
• Data match with inter-band tunneling equation
Source of Holes― Surface Pitting and Holes
[email protected]. Gao et al., IEEE Trans. Electron Devices, vol. 61, no.2, 201424
• Data do not match with impact ionization equationInter-band tunneling Impact ionization
Too small
Traps assist inter-band tunneling
• Traps assist inter-band tunneling causing a lower Et
Source of Holes― Trap-assited inter-band Tunneling
[email protected]. Gao et al., IEEE Trans. Electron Devices, vol. 61, no.2, 201425
Trap Assisted Inter-Band Tunneling ― TCAD Simulations
Vertical Cut at Gate-Drain Edge
Metal Gate
3nm GaN Cap
14nm Al0.28Ga0.72N Barrier
GaN Buffer
Passivation Nitride
Hole Concentration
• TCAD simulation of trap-assisted inter-band tunnelingDonor trap states w. Ec – Et = 0.45eV, Nt = 5x1018 /cm3
Vds = 43 V, Vgs = -7 V
Acknowledgement to our collaborators: Hiu-Yung (Hugh) Wong, Ph.D et al at Synopsys, Inc.
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MtMd
WVTR OH 2
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Water Vapor Transmission Rate (WVTR) of PECVD SiN
≈ 0.05-0.1 g/m2/day
2Alx Ga1-xN + 3H2O = xAl2O3 + (1-x)Ga2O3 + N2 + H2
Estimated WVTR is consistent with the reported value for PECVD SiN (0.01-0.1 g/m2/day) in literature [1-2].
Stressed in wet air Stressed in dry air
Stressed in ambient air Stressed in vacuum
Degradation of SiN Passivation
Defects are created in SiN during off-state stress to accelerate water diffusion.
Last Question: Diffusion of Water― Water Vapor Transmission Rate
[1] A.S. da Silva Sobrinho, et al. J. Vac. Sci. Technol. A, vol. 16, no. 6, pp. 3190-3198, 1998.[2] D.S. Wuu, et al. Surf. Coat. Technol., vol. 198, no.1-3, pp. 114-117, 2004.
[email protected]. Gao et al., IEEE Trans. Electron Devices, vol. 61, no.2, 201427
ConclusionMechanisms of Permanent Degradation
A Water-assisted Field-induced Electrochemical Process
• Cause surface pitting & Ga, Al migration• Cause permanent Id drop
2Alx Ga1-xN + 3H2O = xAl2O3 + (1-x)Ga2O3 + N2 + H2
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Traps assist inter-band tunneling
Is There Impact Ionization ?
Lateral impact ionization
[email protected]. Gao et al., IEEE Trans. Electron Devices, vol. 61, no.2, 201430
• Holes are swept to the source-edge of the gate
Why is there still pits in devices stressed in vacuum ?
In-situ XPS Analysis
Water desorption requires vacuum annealing with T > 200 oC
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[email protected]. Gao et al., IEEE Trans. Electron Devices, vol. 61, no.2, 2014
Source of Holes― Quantitative Analysis:Pits Formation vs Vds
Stressed in Air 1000sVgd = 10V to 60V